This invention relates to low expansion cast iron having an austenitic matrix.
Recently, higher accuracy has become more important for tools and apparatus in the field of electronics, such as machine tools, measuring apparatus and metallic molds, as the field of electronics has been further developed. For example, materials having a coefficient of expansion of at most 4.times.10.sup.-6 /.degree. C. have been demanded for precision instruments.
As a result, some such materials have been developed. These include Invar cast iron (36.5 wt % Ni-Fe cast iron) and Ni-Resist (cast iron of ASTM A439 type D-5), as shown in Table 1.
TABLE 1 __________________________________________________________________________ Thermal expansion Composition (wt %) coefficient C Si Mn Ni Cr Fe (20-200.degree. C.) .times. 10.sup.-6 /.degree.C. __________________________________________________________________________ Ni-Resist &lt;2.40 1.0 2.80 &lt;1.00 34.00 &lt;0.10 balance 5 (ASTM A439) 36.00 Invar -- -- -- 36.5 -- balance 1.2 __________________________________________________________________________
Invar cast iron has a thermal expansion coefficient of 1.2.times.10.sup.-6 /.degree. C., which is a very low coefficient. However, Invar cast iron has a poor castability and is difficult to cut. Thus, its applications are limited.
On the other hand, Ni-Resist has good castability and is easily cut. However, it has a thermal expansion coefficient of about 5.times.10.sup.-6 /.degree. C., which is too high for precision instruments. Accordingly, it cannot meet current demands very well.
Attempts have been made to produce a material which has:
(1) an expansion coefficient not greater than 4.times.10.sup.-6 /.degree. C., and PA0 (2) good castability, good cutting properties and good damping capacity.
However, such a material has not been successfully achieved prior to the present invention.